A large percentage of patients having implantable cardioverter defibrillators also suffer from congestive heart failure (CHF). Increased fluid retention, a compensatory mechanism for increasing the cardiac output of the failing heart, eventually produces high venous pressure leading to pulmonary congestion (or edema) and respiratory dysfunction. Central to the treatment of CHF symptoms, therefore, is the management of a patient's fluid retention status, typically through the use of diuretics. However, patients can become over-diuresed which can cause added stress to the heart condition. Therefore, a careful balance of the patient's fluid status is important in managing the heart failure patient.
Pulmonary congestion/edema and CHF related respiratory disorders are common reasons for hospitalization of CHF patients. Hospitalizations could be reduced with improved monitoring of CHF symptoms such that early warning signs of a deteriorating CHF condition could be recognized and treated proactively. One method proposed for monitoring CHF in ambulatory patients is to measure intra-thoracic impedance to detect pulmonary congestion/edema. Preliminary studies show that measurements of intra-thoracic impedance can be good predictors of impending CHF-related hospitalization.
In U.S. Pat. No. 6,512,949 issued to Combs et al., an impedance monitor for discerning edema through evaluation of respiratory rate is generally disclosed. In U.S. patent application No. 2003/0028221 to Zhu et al., a cardiac rhythm management system having an edema detection circuit that includes a thoracic impedance circuit is generally disclosed. An implantable device for long term monitoring of CHF that includes measurement of systemic venous and pulmonary impedance is generally disclosed in U.S. Pat. No. 6,473,640 issued to Erlebacher.
The use of intra-thoracic impedance measurements for determination of minute ventilation has been implemented in implantable rate-responsive pacemakers. See, for example, U.S. Pat. No. 4,901,725 issued to Nappholz et al., U.S. Pat. No. 4,596,251 issued to Plicchi et al., U.S. Pat. No. 5,562,712 issued to Steinhaus et al., or U.S. Pat. No. 5,562,711 issued to Yerich et al. Methods for measuring intra-thoracic impedance measurements used for determining respiration rate and minute ventilation have been proposed for use in monitoring for pulmonary congestion/edema based on evaluation of respiration rate. Reference is made to commonly-assigned U.S. Pat. No. 5,957,861 issued to Combs et al., and U.S. Pat. No. 5,876,353 issued to Riff.
Intra-thoracic impedance measurements of the type typically used for minute ventilation measurements would require implementation of additional circuitry in an ICD. Generally, to determine minute ventilation using intra-thoracic impedance measurements, the impedance measurement is acquired at a sampling rate that is asynchronous with the heart rate. The use of biphasic excitation pulses has the advantage of delivering a balanced charge pulse to the drive electrode thereby preventing residual charge at the electrode-tissue interface during the relatively high sampling rate. For determining thoracic fluid content, the intra-thoracic impedance measurements need to be sampled at a rate that allows averaging or filtering of the impedance signal associated with cardiac and respiratory cycles.
Lead impedance measurements are known for use in ICDs for monitoring lead and electrode stability. Such measurements are performed using a monophasic excitation pulse that is less than the defibrillation or pacing capture threshold and is delivered during the physiological refractory period of the heart such that the excitation pulse does not capture the heart and is imperceptible to the patient. Reference is made to U.S. Pat. No. 5,755,742 issued to Schuelke et al., and U.S. Pat. No. 6,317,628 issued to Linder et al., both of which patents are incorporated herein by reference in their entirety. The monophasic excitation pulse can be synchronized with cardiac events such that the monophasic pulse occurs during sense amplifier blanking. A single impedance measurement of a particular lead pathway is generally sufficient for lead integrity evaluation.
While lead impedance monitoring methods have been incorporated in ICD systems, such methods have not previously been adapted for use in monitoring intra-thoracic impedance-related changes due to changes in thoracic fluid content. There remains a need, therefore, for a method and apparatus for measuring intra-thoracic impedance in an ICD system for the purpose of monitoring changes in a patient's fluid status. It is desirable that such a method be readily implemented in an ICD without added circuitry or complexity to the device. CHF patient's implanted with ICDs may then be advantageously monitored for changes in fluid status to allow the early detection of pulmonary congestion as well as over-dryness.